1. Field of Invention
The present invention relates to a steering system of an in-wheel motor-driven electric vehicle, and more particularly, to a steering apparatus of an in-wheel motor-driven electric vehicle capable of steering the vehicle by controlling torques applied to in-wheel motors mounted in road wheels, and a steering control method using the steering apparatus.
2. Description of the Prior Art
Due to seriousness of environmental problems and depletion of limited resources caused by use of fossil fuels such as gasoline, diesel and the like, an environment-friendly vehicle such as an electric vehicle, a fuel cell vehicle, or a hybrid vehicle, driven by motors has been developed and driven.
An in-wheel system is a system having individual small motors (in-wheel motors) respectively installed in road wheels to independently and directly control the respective road wheels in an environment-friendly vehicle using electric power as a power source.
The in-wheel system may have a simple driving system as compared with a vehicle having a large motor provided therein and thus have good utilization of space since the individual motors are mounted in the respective road wheels. Further, the in-wheel system may improve behavior performance of the vehicle since the torques for the respective road wheels can be independently controlled by independently and directly controlling the road wheels.
There are also advantages in that steering by driving can be performed by controlling a difference in torque between the left and right road wheels to generate a lateral force and complicated power transfer units such as a transmission and a differential gear may be omitted.
A vehicle using the conventional in-wheel system is mounted with a rack gear, a pinion gear and a steering electric motor for driving the pinion gear as a steering apparatus, and is steered by controlling the electric motor based on sensor information according to user's steering. In addition, a manner of assisting user's steering force through torque outputs of the in-wheel motors has been generalized.
For example, Korean Laid-open Patent Publication No. 2013-0012827 discloses a control apparatus and method of an in-wheel system vehicle, which may provide a straight drive by correcting torques of left and right in-wheel motors when the vehicle is biased to any one side while driving. The in-wheel system disclosed in the above document includes the following procedure: analyzing information on steering angles, a yaw rate, vehicle speed, and steering torque detected in a driving state, determining a bias direction of the vehicle, if it is determined that the vehicle is biased, determining a direction and degree of the bias according to magnitudes of the steering angles and the yaw rate to determine correction torques of the left and right in-wheel motors, and then, controlling the in-wheel motors by driving torques with the correction torques applied thereto, which are determined from the bias of the main driving torque, thereby providing a straight drive.
However, since the conventional in-wheel motor-driven vehicle as described above should be mounted with an additional steering motor as a steering apparatus, there are disadvantages in that due to the additional steering motor, power loss occurs and a production cost is increased.